Reforestation is “our most poorly understood weapon” against climate change, according to Tom Crowther of the Swiss Federal Institute of Technology (ETH Zurich) at the annual meeting of the American Association for the Advancement of Science (AAAS) in Washington, DC.
During his production of the first global map of Earth’s trees in 2015, Crowther located ancient forests that no longer exist but could be replanted. There is room for an additional 1.2 trillion trees outside urban and agricultural areas, he believes. And restoring these forests would be “our most effective weapon in the fight against climate change”.
The amount that reforestation would reduce atmospheric carbon is not yet clear, but the year-old Crowther Lab at ETH plans to address that uncertainty, using a bottom-up approach. The researchers are extrapolating data from millions of on-site observations using artificial intelligence and machine learning, and combining these data with satellite observations to gain “unprecedented insights into the scale of the global forest system and global soil carbon storage”.
The ETH findings will help scientists assess how much carbon could be stored if 1.2 trillion trees were planted to supplement existing forests. Crowther expects this figure would greatly exceed the carbon input reduction from other measures, such as effective refrigeration management worldwide, which has a potential saving of 89 gigatonnes, and plant-only diets for all humans, which could save 66 gigatonnes.
Humans put some 10 gigatonnes of carbon into the atmosphere each year, Crowther said, and in total have increased the atmospheric budget by around 300 gigatonnes.
Complicating the analysis are the roughly 15 billion trees that Crowther estimates are harvested each year, and the loss of soil-sequestered carbon to the atmosphere due to warming in Arctic and boreal regions. The latter releases around 1.5 gigatonnes of carbon annually, Crowther reported, accelerating the rate of climate change by 12–17%.
Biodiversity datasets from the tree-counting and related experiments are being made public, Crowther says, which should help scientists and policy-makers develop effective global-scale targets and identify the most critical regions to restore ecosystems and protect soils. Already, under the auspices of the United Nations, 17 billion trees have been restored in high carbon-capture areas over the past three years, he added.
The 2015 map revealed that Earth currently hosts 3.04 trillion trees, rather than the 400 billion of previous estimates. The discrepancy arose because earlier data were based on satellite imagery alone. These data noted the location of forests but gave no information on the structure below the forest canopy or on biodiversity, according to Crowther.
Crowther’s larger estimate was based on more than a million ground-sourced measurements of trees worldwide, that were then scaled up using satellite data.
